1. Field of the Invention
The present invention relates to automatic aircraft control systems including autopilot and autothrottle channels, particularly with respect to simultaneously controlling vertical speed or flight path angle and airspeed or Mach.
2. Description of the Prior Art
Present day automatic aircraft control system perform simultaneous control of vertical speed (or flight path angle) and airspeed (or Mach) by utilizing the pitch channel of the autopilot to control vertical speed or flight path angle and the autothrottle channel to control airspeed or Mach. Such prior art automatic flight control systems have the problem that an excessive vertical climb speed command or flight path angle command may result in a decrease of airspeed to an unsafe value.
Present day autothrottle systems are conventionally equipped with apparatus to limit the control authority of the throttles to prevent overboosting and consequently damaging the engines. Modern autopilot systems include apparatus that limits the pitch attitude attainable by the aircraft to prevent overstressing the airframe and causing discomfort to the passengers. The prior art automatic flight control systems of the type described have an insidious characteristic whereby a vertical climb speed or flight path angle that is reasonable and achievable at low altitudes will become excessive as the aircraft climbs because the thrust capability of the engines decreases as the aircraft altitude increases and the throttles eventually attain the built-in authority limit as the autothrottle channel endeavors to maintain the airspeed command. When the throttles attain the upper limited value, the airspeed cannot be maintained and begins to decrease as the autopilot endeavors to maintain the vertical speed or flight path angle by increasing the pitch attitude of the aircraft until the autopilot attains the built-in pitch limit. Consequently, in such prior art automatic flight control systems under such conditions, neither the vertical speed (or flight path angle) nor the airspeed (or Mach) remains under control and both values decrease. These dangerous conditions are exacerbated because of the increased drag resulting from the high angle of attack of the aircraft that is induced by a large pitch attitude.
Additionally, such prior art automatic flight control systems are prone to providing an unsafe and uncontrolled descent airspeed when the throttles contact the flight idle position if an unreasonably large descent vertical speed or flight path angle is commanded.